The present invention relates to an aluminum vacuum brazing furnace and, more specifically, to an aluminum vacuum brazing furnace which is used for brazing aluminum members constituting an aluminum heat exchanger, such as an automobile radiator, an automobile air-conditioning evaporator, a condenser or the like.
An aluminum vacuum brazing furnace is conventionally known as disclosed in Japanese Patent Unexamined Publication No. 63-52764.
Objective of the vacuum brazing furnace disclosed in the above publication is to prevent deterioration of the vacuum atmosphere in the heating chamber due to magnesium vapor from the brazing metal. In order to achieve this objective, the above-described conventional aluminum vacuum brazing furnace has heating chamber formed within the furnace body. A gas ventilating hole, through which the inside and the outside of the heating chamber are in communication is formed in a portion of a wall defining the heating chamber. The gas ventilating hole can be opened and closed by a movable wall so that, when the temperature in the heating chamber reaches approximately the magnesium evaporation temperature, the gas ventilating hole is closed by the movable wall so as to seal the evaporated magnesium in the heating chamber, thereby preventing magnesium from scattering to the vacuum exhaust system and the like.
However, the present inventors have noted that the evaporated magnesium adheres to a sealing portion between the movable wall and the gas ventilating hole to hinder the perfect sealing of the gas ventilating whole. Accordingly, the evaporated magnesium in the heating chamber scatters from within the heating chamber to the exhaust pump system, the magnesium content in the brazing metal must be relatively high and there gives rise to an irregular motion of the movable wall.
As a result of investigation by the present inventors, the above-described problems occur for the following reason. Namely, according to the prior art, since the temperature of the furnace body is relatively low (about 70.degree..about.80.degree. C. and since the movable wall is not provided with a heater, then when the gas ventilating hole is not closed by the movable wall, the temperature of the movable wall is affected by the low temperature of the furnace body temperature so as to become lower than a temperature (400.degree. C.) at which the evaporated magnesium can adhere. Accordingly when the movable wall is moved so as to close the gas ventilating hole, the evaporated magnesium adheres to the movable wall of low temperature.
Generally, the conventional aluminum vacuum brazing furnace is so constructed that, after the aluminum members combined with the magnesium containing brazing metal are carried into the heating chamber, the heating chamber is evacuated to form a vacuum atmosphere and is heated to above a melting point of the brazing metal so as to braze the aluminum members (see, for example, Japanese Patent Unexamined Publication No. 63-52764). Here, the reason why the brazing metal contains magnesium is that it is possible to break the oxide film of aluminum during the brazing to perform the brazing satisfactorily.
However, the present inventors have found that, after finishing the conventional brazing process, when the brazed aluminum works are taken from the heating chamber and aluminum members which are to be brazed are carried in the heating chamber, the heating chamber is exposed to the atmospheric air so that the evaporated magnesium in the heating chamber is oxidized to become powdered magnesium oxide. The magnesium oxide powder in turn accumulates on the floor surface of the heating chamber, and the magnesium oxide thus accumulated is typically scattered into the vacuum exhaust system at the time of getting a vacuum for the brazing cycle, resulting in the irregular operation, deterioration of performance, clogging of filter and the like of the pumps used for performing the vacuum evacuation. Further, in order to prevent such problems, a cleaning operation to remove the accumulation of magnesium oxide must performed frequently by hand. Moreover, since evaporated magnesium which is not yet oxidized is present in the heating chamber and since the impact energy is large in general when the evaporated magnesium is oxidized, the heating chamber is very dangerous. Accordingly the cleaning operation must be performed carefully paying attention to this fact.
The conventional aluminum vacuum brazing furnace is so constructed that after the aluminum members combined with the brazing metal containing magnesium are carried in the heating chamber within the furnace body, the heating chamber is evacuated to form a vacuum atmosphere and the brazing metal is heated to above the melting point thereof to braze the aluminum members. The heating chamber is therefore provided with a heater for heating the brazing metal to above the melting point thereof.
With regard to heating the brazing metal, it is desirable to heat the razing metal uniformly. However, in order to heat the brazing metal uniformly, it is necessary to arrange the heater at a relatively long distance from the brazing metal. Accordingly, the heater must generate a sufficiently high output power to heat the brazing metal even from a relatively long distance. According to such demand, the main current of the conventional heater was the strip heater of Fe--Cr--Ni group in which a heating element and electrode units (including insulators) connected to both ends of the heating member are exposed.
However, the present inventors have found that in the above-described conventional heater, magnesium evaporated from the brazing metal during the vacuum brazing adheres to low-temperature portions including the insulator of the electrode unit, thereby reducing the insulation resistance of the insulator and risking a short-circuit accident in the heater.
Further, the present inventors have found that when the heating chamber is exposed to atmospheric air after taking out the brazed aluminum members, evaporated magnesium floating in the heating chamber is oxidized to become powdered magnesium oxide which in turn scatters and adheres to the heating member, the electrode units and the like. Accordingly, it is necessary to perform a cleaning operation to remove the magnesium oxide adhered to the heating member. However, complicated configurations of the heating member, the electrode units makes the cleaning operation troublesome.